Image sensor having stacked conformal films

ABSTRACT

An image sensor device is disclosed. The image sensor device includes: a substrate having a front surface and a back surface; a radiation-sensing region formed in the substrate; an opening extending from the back surface of the substrate into the substrate; a first metal oxide film including a first metal, the first metal oxide film being formed on an interior surface of the opening; and a second metal oxide film including a second metal, the second metal oxide film being formed over the first metal oxide film; wherein the electronegativity of the first metal is greater than the electronegativity of the second metal. An associated fabricating method is also disclosed.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of application Ser. No. 15/072,887, filedon Mar. 17, 2016. All of the above-referenced applications are herebyincorporated herein by reference in their entirety.

BACKGROUND

Semiconductor image sensors are used for sensing light. Complementarymetal-oxide-semiconductor (CMOS) image sensors (CIS) and charge-coupleddevice (CCD) sensors are widely used in various applications such asdigital still camera or mobile phone camera applications. These devicesutilize an array of pixels in a substrate, including photodiodes andtransistors, that can absorb radiation projected toward the substrateand convert the sensed radiation into electrical signals.

A back side illuminated (BSI) image sensor device is one type of imagesensor device. As transistor device size shrinks with each technologygeneration, existing BSI image sensor devices may begin to suffer fromissues related to cross-talk and blooming. These issues may be caused byinsufficient isolation between neighboring pixels of the BSI imagesensor.

Therefore, while existing methods of fabricating BSI image sensordevices have been generally adequate for their intended purposes, theyhave not been entirely satisfactory in every aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 to FIG. 7 are cross-sectional views of an image sensor devicefabricated at various operations, in accordance with some embodiments ofthe present disclosure;

FIG. 8 is a schematic view showing the experiment results of testing thenumber of white pixels of devices fabricated according to the processesin the prior arts and the exemplary embodiment of FIG. 4A; and

FIG. 9 is a schematic view showing the experiment results of testing thenumber of white pixels of devices fabricated according to the processesin the prior arts and the embodiment of FIG. 4B.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

Notwithstanding that the numerical ranges and parameters setting forththe broad scope of the disclosure are approximations, the numericalvalues set forth in the specific examples are reported as precisely aspossible. Any numerical value, however, inherently contains certainerrors necessarily resulting from the standard deviation found in therespective testing measurements. Also, as used herein, the term “about”generally means within 10%, 5%, 1%, or 0.5% of a given value or range.Alternatively, the term “about” means within an acceptable standarderror of the mean when considered by one of ordinary skill in the art.Other than in the operating/working examples, or unless otherwiseexpressly specified, all of the numerical ranges, amounts, values andpercentages such as those for quantities of materials, durations oftimes, temperatures, operating conditions, ratios of amounts, and thelikes thereof disclosed herein should be understood as modified in allinstances by the term “about.” Accordingly, unless indicated to thecontrary, the numerical parameters set forth in the present disclosureand attached claims are approximations that can vary as desired. At thevery least, each numerical parameter should at least be construed inlight of the number of reported significant digits and by applyingordinary rounding techniques. Ranges can be expressed herein as from oneendpoint to another endpoint or between two endpoints. All rangesdisclosed herein are inclusive of the endpoints, unless specifiedotherwise.

The image sensor device according to the present disclosure is abackside-illuminated (BSI) image sensor device. The BSI image sensordevice includes a charge-coupled device (CCD), a complementary metaloxide semiconductor (CMOS) image sensor (CIS), an active-pixel sensor(APS) or a passive-pixel sensor. The image sensor device may includeadditional circuitry and input/outputs that are provided adjacent to thegrid of pixels for providing an operation environment of the pixels andfor supporting external communication with the pixels.

FIG. 1 to FIG. 7 are cross-sectional views of an image sensor devicefabricated at various operations, in accordance with some embodiments ofthe present disclosure. It is understood that FIG. 1 to FIG. 7 have beensimplified for a better understanding of embodiments of the presentdisclosure.

Referring to FIG. 1, the image sensor device 100 includes a substrate102. The substrate 102 is a device substrate. The substrate 102 may be asemiconductor substrate. The substrate 102 may be a silicon substratedoped with a P-type dopant such as boron, in which case the substrate102 is a P-type substrate. Alternatively, the substrate 102 could beanother suitable semiconductor material. For example, the substrate 102may be a silicon substrate doped with an N-type dopant such asphosphorous or arsenic, in which case the substrate is an N-typesubstrate. The substrate 102 may include other elementary semiconductormaterials such as germanium or diamond. The substrate 102 may optionallyinclude a compound substrate and/or an alloy semiconductor. Further, thesubstrate 102 may include an epitaxial layer (epi layer), may bestrained for performance enhancement, and may include asilicon-on-insulator (SOI) structure.

The substrate 102 has a front surface 104 (also referred to as afrontside) and a back surface 106 (also referred to as a backside). Fora BSI image sensor device such as the image sensor device 100, incidentradiation enters the substrate 102 through the back surface 106. In someembodiments, the substrate 102 has a thickness ranging from about 500 μmto about 100 μm. The substrate 102 is fabricated with front endprocesses, in accordance with some embodiments. For example, thesubstrate 102 includes various regions, which may include a pixelregion, a periphery region, a bonding pad region, and a scribe lineregion. For the sake of simplicity, only a portion of the pixel regionis shown in FIGS. 1 to 7.

The pixel region includes radiation-sensing regions 108 and dopedisolation regions 110. The radiation-sensing regions 108 are doped witha doping polarity opposite from that of the substrate 102. Theradiation-sensing regions 108 are formed by one or more implantationprocesses or diffusion processes. The radiation-sensing regions 108 areformed adjacent to or near the front surface 104 of the substrate 102.Although only a portion of the pixel region is shown in FIG. 1, thepixel region may further include pinned layer photodiodes, photodiodegates, reset transistors, source follower transistors, and transfertransistors. For the sake of simplicity, detailed structures of theabove features are not shown in figures of the present disclosure.

The radiation-sensing regions 108 are operable to sense incidentradiation that enters the pixel region from the back surface 106. Theincident radiation may be visual light. Alternatively, the incidentradiation may be infrared (IR), ultraviolet (UV), X-ray, microwave,other suitable types of radiation, or a combination thereof.

The doped isolation regions 110 are adjacent to the radiation-sensingregions 108, in accordance with some embodiments. The doped isolationregions 110 are formed adjacent to or near the front surface 104. Eachpair of neighboring radiation-sensing regions 108 is separated from oneanother by one of the respective doped isolation regions 110. The dopedisolation regions 110 are doped with a doping polarity the same as thatof the substrate 102. In some embodiments, the doping concentration ofthe doped isolation regions 110 is higher than that of the substrate102. For example, the doping concentration of the doped isolationregions 110 may be in a range of about 1E16 per cm³ to about 1E20 percm³. The doped isolation regions 110 are formed by one or moreimplantation processes or diffusion processes.

As shown in FIG. 1, isolation features 112 are formed in the dopedisolation regions 110, in accordance with some embodiments. Theisolation features 112 are formed adjacent to or near the front surface104 of the substrate 102. In some embodiments, the isolation features112 are used to define predetermined regions of the radiation-sensingregions 108 and doped isolation regions 110. Therefore, the isolationfeatures 112 may be formed before forming the radiation-sensing regions108 and doped isolation regions 110. In some embodiments, the dopedisolation regions 110 are aligned with the isolation features 112.

The isolation features 112 include shallow trench isolation (STI)structures and/or local oxidation of silicon (LOCOS) structures. In someembodiments, some active or passive features, such as MOSFET or junctioncapacitor, are formed in the doped isolation regions 110, according todesign needs and manufacturing concerns. The active or passive featuresin the doped isolation regions 110 are surrounded and protected by theisolation features 112. The thickness of the isolation features 112 isgreater than that of the active or passive features in the dopedisolation regions 110. In some embodiments, the thickness of theisolation features 112 is in a range from about 100 angstroms to about5000 angstroms.

In some embodiments, the isolation features 112 are formed by formingtrenches in the substrate 102 from the front surface 104 and filling adielectric material into the trenches. The dielectric material mayinclude silicon oxide, silicon nitride, silicon oxynitride, a low-kmaterial, or another suitable dielectric material. A chemical mechanicalpolishing (CMP) process may be performed to planarize the surface of thedielectric material filling the trenches.

As shown in FIG. 1, the image sensor device 100 may further include aninterconnection structure 114 formed over the front surface 104 of thesubstrate 102. The interconnection structure 114 includes a number ofpatterned dielectric layers and conductive layers that couple to variousdoped features, circuitry, and input/output of the image sensor device100. The interconnection structure 114 includes an interlayer dielectric(ILD) and a multilayer interconnection (MLI) structure. The MLIstructure includes contacts, vias and metal lines. For the purpose ofillustration, a number of conductive lines 116 and vias/contacts 118 areshown in FIG. 1, it being understood that the conductive lines 116 andvias/contacts 118 are merely exemplary. The actual positioning andconfiguration of the conductive lines 116 and vias/contacts 118 may varydepending on design needs and manufacturing concerns.

Referring to FIG. 2, a buffer layer 120 is formed on the interconnectionstructure 114, in accordance with some embodiments. The buffer layer 120may include a dielectric material such as silicon oxide. Alternatively,the buffer layer 120 may include silicon nitride. The buffer layer 120may be deposited by chemical vapor deposition (CVD), physical vapordeposition (PVD), or other suitable techniques. The buffer layer 120 maybe planarized to form a smooth surface by a CMP process.

Afterwards, a carrier substrate 122 is bonded with the substrate 102through the buffer layer 120. Therefore, the processing of the backsurface 106 of the substrate 102 can be performed. In some embodiments,the carrier substrate 122 is similar to the substrate 102 and includes asilicon material. Alternatively, the carrier substrate 122 may include aglass substrate or another suitable material. The carrier substrate 122may be bonded to the substrate 102 by molecular forces (direct bonding),optical fusion bonding, metal diffusion bonding, anodic bonding, or byother suitable bonding techniques. The buffer layer 120 provideselectrical isolation between the substrate 102 and carrier substrate122. The carrier substrate 122 provides protection for the variousfeatures formed on the front surface 104 of the substrate 102. Thecarrier substrate 122 also provides mechanical strength and support forprocessing the back surface 106 of the substrate 102 as discussed below.

After the carrier substrate 122 is bonded, a thinning process is thenperformed to thin the substrate 102 from the back surface 106. Thethinning process may include a mechanical grinding process. Afterwards,an etching chemical may be applied over the back surface 106 ofsubstrate 102 to further thin the substrate 102 to a thickness which ison the order of a few microns. In some embodiments, the thickness of thesubstrate 102, after being thinned, is in a range from about 1 μm toabout 100 μm.

Common image sensor device defects include optical cross-talk,electrical cross-talk and dark current. The defects become more seriousas the image pixel sizes and the spacing between neighboring imagepixels continues to shrink. Optical cross-talk refers to photoninterference from neighboring pixels that degrades the light-sensingreliability and accuracy of the pixels. Dark current may be defined asthe existence of pixel current when no actual illumination is present.In other words, the dark current is the current that flows through thephotodiode when no photons are entering the photodiode. White pixelsoccur where an excessive amount of current leakage causes an abnormallyhigh signal from the pixels. In the image sensor device 100 shown inFIG. 2, the doped isolation regions 110 have a doping polarity oppositeto that of the radiation-sensing regions 108 to reduce the dark currentand white pixel defects. However, the doped isolation regions 110 alonemay not effective enough to prevent dark current and white pixeldefects. In addition, the doped isolation regions 110 could not resolvethe optical cross-talk defect due to the similar refractive index of theradiation-sensing regions 108 and doped isolation regions 110.

Referring to FIG. 3, an etching process is performed on the back surface106 of the substrate 102 to form a number of openings 124 (ortrenches/recesses). The etching process includes a dry etching process.An etching mask (for example a hard mask, not illustrated herein) may beformed before the etching process is performed. Each of the openings 124has a width W1 at the back surface 106 of the substrate 102. The widthW1 may be smaller than or substantially equal to that of the dopedisolation regions 110. The openings 124 may have a rectangular shape, atrapezoidal shape, or other suitable shape. In some embodiments, each ofthe openings 124 extends over half of the thickness of the substrate 102but does not reach the isolation features 112. Accordingly, active orpassive features surrounded by the isolation features 112 may be notdamaged by the etching process. In some embodiments, the depth of theopenings 124, measured from the back surface 106 of the substrate 102,is in a range from about 1 μm to about 10 μm. The depth of the openings124 may be adjusted by time control without using an etching stop layer.These openings 124 are used for forming deep-trench isolation (DTI)structures, which will be discussed in more detail below. At thecompletion of the openings 124 formation, an interior surface 124′ ofthe DTI structure in the substrate 102 is exposed.

Referring to FIG. 4A, a high-k film 126 is formed over the interiorsurface 124′ of the DTI structure, in accordance with an exemplaryembodiment of the present disclosure. In some embodiments, as can beseen from the locally enlarged portion of the high-k film 126, a thininterlayer 125 consisting of e.g. SiO₂ can be applied between thesubstrate 102 and the high-k film 126 as an adhesion-promoting layer.The thickness of the interlayer 125 may be preferably less than about 25angstroms. In some embodiments, the thickness of the interlayer 125 maybe about 20 angstroms. The high-k film 126 may further cover the backsurface 106. In the exemplary embodiment, the high-k film 126effectively possesses a greater overall surface negative charge at oneside of the interior surface 124′ than that of traditional dielectricfilms. The effective surface negative charge induces effective surfacepositive charges at the other side of the interior surface 124′ of theDTI structure. The induced effective surface positive charges annihilatenegatively-charged crystal defects inherently residing in the proximityof the interior surface 124′ due to damages made during the opening 124formation. Hence, such arrangement of the high-k film 126 reduces darkcurrent and/or white pixels in an image sensor device 100.

According to one or more embodiments, the high-k film 126 is a high-kmetal oxide including an XO high-k layer and a YO high-k layer. X and Yare two elements on periodic table, O is oxygen. In particular, the XOhigh-k layer and the YO high-k layer may be a combination of at leasttwo of a hafnium oxide, aluminum oxide, zirconium oxide, magnesiumoxide, calcium oxide, yttrium oxide, tantalum oxide, strontium oxide,titanium oxide, lanthanum oxide, barium oxide or other metal oxides thatcan form a high-k film using existing semiconductor depositiontechnologies. In addition, the sequential arrangement of the XO high-klayer and the YO high-k layer is determined in accordance to theelectronegativities of the X and the Y with respect to their oxide form.For example, the element among X and Y having greater electronegativityis disposed closer to the substrate 102. On the other hand, the elementamong X and Y having lower electronegativity is disposed farther fromthe substrate 102. In this connection, the high-k film 126 demonstratesan electronegativity gradient with compounds having higherelectronegativity element closer to the substrate 102 and compoundshaving lower electronegativity element farther from the substrate 102.

In the exemplary embodiment, the gradient high-k film 126 is comprisedof an aluminum oxide (Al₂O₃) layer 126_1 and a hafnium oxide (HfO₂)layer 126_2. In some embodiments, the thickness of the aluminum oxide(Al₂O₃) layer 126_1 may be greater than about 30 angstroms. In someembodiments, the thickness of the aluminum oxide (Al₂O₃) layer 126_1 maybe about 60 angstroms. In some embodiments, the thickness of the hafniumoxide (HfO₂) layer 126_2 may be greater than about 30 angstroms. In someembodiments, the thickness of the hafnium oxide (HfO₂) layer 126_2 maybe about 60 angstroms.

The aluminum in the aluminum oxide (Al₂O₃) layer 126_1 has a strongertendency to attract electrons (or electron density) towards itself thanthe hafnium in the hafnium oxide (HfO₂) layer 126_2 does. The gradientconfiguration of high-k layers can help to mitigate crystal defectscausing dark current and white pixel. In particular, the disclosedgradient high-k film 126 having the configuration of the high-k layersalternatingly arranged in a sequentially stacked manner (i.e. thealuminum oxide (Al₂O₃) layer 126_1 on the thin interlayer 125 andfurther the hafnium oxide (HfO₂) layer 126_2 on the aluminum oxide(Al₂O₃) layer 126_1) can dramatically reduce the dark current and whitepixel compared to the existing single layer dielectric film.

FIG. 8 is a schematic view showing the experiment results of testing thenumber of white pixels of devices fabricated according to the processesin the prior arts and the exemplary embodiment of FIG. 4A. From FIG. 8,it can be known that compared with the prior arts in which a singlelayer of hafnium oxide (HfO₂) or aluminum oxide (Al₂O₃) is formed overthe back surface of the image sensor device, the gradient high-k film126 comprised of the hafnium oxide (HfO₂) layer 126_2 and the aluminumoxide (Al₂O₃) layer 126_1 significantly reduces the number of the whitepixels, thus greatly improving the pixel performance of the image sensordevice 100.

The high-k metal oxide may be deposited using a CVD process or a PVDprocess. The CVD process may be plasma enhanced chemical vapordeposition (PECVD) including ICPECVD, a low pressure chemical vapordeposition (LPCVD), or an atomic layer deposition (ALD) with or withoutplasma. These processes may be tuned to favor an accumulation ofnegative charge by varying the process parameters including various flowrates and power parameters, and may involve a treatment step after thefilm deposition to increase negative charge. The resulting high-k metaloxide film may have an oxygen-rich composition with negatively chargedinterstitial oxygen atoms and/or dangling/broke metal oxide bonds, bothof which results in a cumulated negative charge.

FIG. 4B illustrates the gradient high-k film 126 being formed over theback surface 106 of substrate 102 and the interior surface 124′ of theDTI structure, in accordance with another embodiment of the presentdisclosure. Similar to FIG. 4A, as can be seen from the locally enlargedportion of the gradient high-k film 126, the thin interlayer 125consisting of e.g. SiO₂ can be applied between the substrate 102 and thegradient high-k film 126 as adhesion-promoting layer. The thickness ofthe interlayer 125 may be preferably less than about 25 angstroms. Insome embodiments, the thickness of the interlayer 125 may be about 20angstroms. The gradient high-k film 126 may conformally cover the backsurface 106, including covering interior surfaces 124′ of the openings124 in a conformal manner. The gradient high-k film 126 is comprised ofthe aluminum oxide (Al₂O₃) layer 126_1, the hafnium oxide (HfO₂) layer126_2 and further a tantalum oxide (Ta₂O₅) layer 126_3. In someembodiments, the thickness of the aluminum oxide (Al₂O₃) layer 126_1 maybe greater than about 10 to 30 angstroms. In some embodiments, thethickness of the aluminum oxide (Al₂O₃) layer 126_1 may be about 31 to60 angstroms. In some embodiments, the thickness of the hafnium oxide(HfO₂) layer 126_2 may be about 10 to 30 angstroms. In some embodiments,the thickness of the hafnium oxide (HfO₂) layer 126_2 may be about 31 to60 angstroms. In some embodiments, the thickness of the tantalum oxide(Ta₂O₅) layer 126_3 may be greater than about 10 to 30 angstroms. Insome embodiments, the thickness of the tantalum oxide (Ta₂O₅) layer126_3 may be about 31 to 60 angstroms.

The tantalum oxide (Ta₂O₅) layer 126_3 has a refractive index of about2.2, which is greater than a refractive index (about 2) of the hafniumoxide (HfO₂) layer 126_2; and the refractive index of the hafnium oxide(HfO₂) layer 126_2 is also greater than a refractive index (about 1.6)of the aluminum oxide (Al₂O₃) layer 126_1. Moreover, the refractiveindex of the aluminum oxide (Al₂O₃) layer 126_1 is also greater than arefractive index (about 1.4-1.5) of the thin interlayer 125. The stackedlayers of the thin interlayer 125, the aluminum oxide (Al₂O₃) layer126_1, the hafnium oxide (HfO₂) layer 126_2 and the tantalum oxide(Ta₂O₅) layer 126_3 commonly form a gradient anti-reflective coating(ARC) in which layers of high-refractive index material andlow-refractive-index material are alternatingly arranged in asequentially stacked manner. The gradient high-k film 126 significantlyincreases the quantum efficiency (QE), light quality, light quantityinto the radiation-sensing regions 108, and reduces the opticalcross-talk between pixels.

FIG. 9 is a schematic view showing the experiment results of testing thenumber of white pixels of devices fabricated according to the processesin the prior arts and the embodiment of FIG. 4B. From FIG. 9, it can beknown that compared with the prior arts, the gradient high-k film 126comprised of the tantalum oxide (Ta₂O₅) layer 126_3, the hafnium oxide(HfO₂) layer 126_2 and the aluminum oxide (Al₂O₃) layer 126_1significantly reduces the reflectance in the wavelength band of about400 nm to about 600 nm, thus greatly increasing the QE and reduces theoptical cross-talk between pixels.

Afterwards, referring to FIG. 5, a dielectric material 128 is depositedover the back surface 106 of the substrate 102, in accordance with someembodiments. The dielectric material 128 fills the remaining spaces ofthe openings 124. In some embodiments, the dielectric material 128includes silicon oxide, silicon nitride, silicon oxynitride, spin onglass (SOG), low-k dielectric, or another suitable dielectric material.The dielectric material 128 may be deposited by CVD, PVD, or anothersuitable depositing technique. In some embodiments, a portion of thedielectric material 128 outside the openings 124 is thinned andplanarized. In the following discussion, the openings 124 and portionsof the gradient high-k film 126 and dielectric material 128 in theopenings 124 are collectively referred to as deep-trench isolationstructures 130.

In some embodiment, a reflective grid (not shown) may be formed over thesubstrate 102 in order to prevent incident radiation from traveling intothe deep-trench isolation structures thus reducing optical cross-talkdefect. For example, the reflective grid may be formed on the dielectricmaterial 128. Each piece of the reflective grid may be aligned with oneof the respective deep-trench isolation structures 130. In someembodiments, the reflective grid may be formed of a metal material, suchas aluminum, tungsten, copper, tantalum, titanium, alloys thereof, orcombinations thereof. Each piece of the reflective grid may have arectangular shape, a reverse trapezoidal shape, reverse triangle shape,or another suitable shape. The reflective grid may be formed by asuitable deposition process and then patterned. The deposition processincludes electroplating, sputtering, CVD, PVD or other suitabledepositing techniques. The CVD process may be a PECVD including ICPECVD,an LPCVD, or an ALD with or without plasma.

Afterwards, referring to FIG. 6, a transparent filling layer 134 isdeposited over the back surface 106 of the substrate 102, in accordancewith some embodiments. The transparent filling layer 134 may be made ofsilicon oxide, silicon nitride, or suitable polymers, and may be formedby suitable techniques, such as CVD, PVD, or combinations thereof. Insome embodiments, the transparent filling layer 134 has a thicknessranging from about 10 angstroms to about 1000 angstroms. In someembodiments, the transparent filling layer 134 functions as anantireflective layer of the image sensor device 100. The antireflectivelayer serves to reduce reflection of the incident radiation projectedtoward the back surface 106 of the image sensor device 100.

Thereafter, referring to FIG. 7, a color filter layer 136 is formed overthe transparent filling layer 134, in accordance with some embodiments.The color filter layer 136 supports the filtering of incident radiationhaving a particular range of wavelengths, which may correspond to aparticular color of light, for example, red, green, or blue. The colorfilter layer 136 may be used to allow only light having a predeterminedcolor to reach of the radiation-sensing regions 108. Afterwards, a microlens layer 138 may be formed over the color filter layer 136 fordirecting incident radiation toward the radiation-sensing regions. Themicro lens layer 138 may be positioned in various arrangements and havevarious shapes depending on the refractive index of the material usedfor the micro lens layer 138 and/or the distance between the micro lenslayer 138 and the radiation-sensing regions 108. Alternatively, theposition of the color filter layer 136 and micro lens layer 138 may bereversed such that the micro lens layer 138 may be disposed between theback surface 106 of the substrate 102 and color filter layer 138.

Embodiments of mechanisms for forming an image sensor device aredescribed. The gradient high-k film comprised of materials of differentnegative charges and refractive indexes alternatingly arranged in asequentially stacked manner, can significantly reduce the dark currentand white pixel defects, and further improve the QE.

Some embodiments of the present disclosure provide an image sensordevice. The image sensor device includes: a substrate having a frontsurface and a back surface; a radiation-sensing region formed in thesubstrate; an opening extending from the back surface of the substrateinto the substrate; a first metal oxide film including a first metal,the first metal oxide film being formed on an interior surface of theopening; and a second metal oxide film including a second metal, thesecond metal oxide film being formed over the first metal oxide film;wherein the electronegativity of the first metal is greater than theelectronegativity of the second metal.

Some embodiments of the present disclosure provide an image sensordevice. The image sensor device includes: a substrate having a frontsurface and a back surface; a radiation-sensing region formed in thesubstrate; an opening extending from the back surface of the substrateinto the substrate: and a film having gradient refractive indexes overan interior surface of the opening; wherein the film includes aplurality of layers alternatingly arranged in a sequentially stackedmanner according to refractive indexes, and a layer of the plurality oflayers closer to the substrate has a lower refractive index than a layerof the plurality of layers farther from the substrate does.

Some embodiments of the present disclosure provide a method offabricating an image sensor device. The method includes: providing asubstrate having a front surface and a back surface; forming aradiation-sensing region adjacent to the front surface; forming anopening in the substrate from the back surface; and forming a firstmetal oxide film including a first metal on an interior surface of theopening; forming a second metal oxide film including a second metal overthe first metal oxide film; wherein the electronegativity of the firstmetal is greater than the electronegativity of the second metal.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother operations and structures for carrying out the same purposesand/or achieving the same advantages of the embodiments introducedherein. Those skilled in the art should also realize that suchequivalent constructions do not depart from the spirit and scope of thepresent disclosure, and that they may make various changes,substitutions, and alterations herein without departing from the spiritand scope of the present disclosure.

Moreover, the scope of the present application is not intended to belimited to the particular embodiments of the process, machine,manufacture, composition of matter, means, methods and steps describedin the specification. As one of ordinary skill in the art will readilyappreciate from the disclosure of the present invention, processes,machines, manufacture, compositions of matter, means, methods, or steps,presently existing or later to be developed, that perform substantiallythe same function or achieve substantially the same result as thecorresponding embodiments described herein may be utilized according tothe present invention. Accordingly, the appended claims are intended toinclude within their scope such processes, machines, manufacture,compositions of matter, means, methods, or steps.

What is claimed is:
 1. An image sensor device, comprising: a substratehaving a front surface and a back surface; two adjacentradiation-sensing regions formed in the substrate; an opening extendingfrom the back surface of the substrate into the substrate; a trenchisolation feature formed of dielectric material extending from the frontsurface of the substrate into the substrate between the two adjacentradiation-sensing regions; a first metal oxide film including a firstmetal, the first metal oxide film being formed on an interior surface ofthe opening; a second metal oxide film including a second metal, thesecond metal oxide film being formed over the first metal oxide film;and a third metal oxide film including a third metal, the third metaloxide film being formed over the second metal oxide film; wherein theelectronegativity of the first metal is greater than theelectronegativity of the second metal, and the electronegativity of thesecond metal is greater than the electronegativity of the third metal,and each of the first metal oxide film, the second metal oxide film, andthe third metal oxide film is stacked up from the substrate in acontiguous and conformal manner.
 2. The image sensor device of claim 1,wherein the isolation feature abuts the front surface, and the firstmetal oxide film is conformally formed on an interior surface of theopening.
 3. The image sensor device of claim 1, wherein the first metalincludes aluminum.
 4. The image sensor device of claim 1, wherein thesecond metal includes hafnium.
 5. The image sensor device of claim 1,wherein the opening has a depth greater than 1.5 um.
 6. The image sensordevice of claim 1, wherein a thickness of each of the first metal oxidefilm and the second metal oxide film is greater than 30 Angstroms. 7.The image sensor device of claim 1, wherein the first metal oxide filmis further conformally disposed over the back surface of the substrate.8. The image sensor device of claim 1, further comprising an interlayerbetween the first metal oxide film and the interior surface of theopening.
 9. The image sensor device of claim 8, wherein the interlayerincludes SiO₂.
 10. The image sensor device of claim 8, wherein athickness of the interlayer is less than 25 μm.
 11. The image sensordevice of claim 1, further comprising a tantalum oxide (Ta₂O₅) layerformed over the second metal oxide film.
 12. An image sensor device,comprising: a substrate having a front surface and a back surface; twoadjacent radiation-sensing regions formed in the substrate; a trenchisolation feature formed of dielectric material extending from the frontsurface of the substrate into the substrate between the two adjacentradiation-sensing regions and abutting the front surface; a trenchextending from the back surface of the substrate into the substrate; afirst film including a first metal, the first film being formed on aninterior surface of the trench; a second film including a second metal,the second film being formed over the first film; and a third filmincluding a third metal, the third film being formed over the secondfilm; wherein the electronegativity of the first metal is greater thanthe electronegativity of the second metal, and the electronegativity ofthe second metal is greater than the electronegativity of the thirdmetal, and each of the first film, the second film, and the third filmis stacked up from the substrate in a contiguous and conformal manner.13. The image sensor device of claim 12, wherein the first film isconformally formed on an interior surface of the trench.
 14. The imagesensor device of claim 12, wherein the first metal includes aluminum.15. The image sensor device of claim 12, wherein the second metalincludes hafnium.
 16. The image sensor device of claim 12, wherein thetrench has a depth greater than 1.5 um.
 17. The image sensor device ofclaim 12, wherein a thickness of each of the first film and the secondfilm is greater than 30 Angstroms.
 18. The image sensor device of claim12, wherein the first film is further conformally disposed over the backsurface of the substrate.
 19. An image sensor device, comprising: asubstrate having a front surface and a back surface; two adjacentradiation-sensing regions formed in the substrate; a trench extendingfrom the back surface of the substrate into the substrate; a trenchisolation feature formed of dielectric material extending from the frontsurface of the substrate into the substrate between the two adjacentradiation-sensing regions; and a plurality of metal oxide filmssequentially stacked up on an interior surface of the trench in acontiguous and conformal manner, wherein a first metal oxide film of theplurality of metal oxide films has greater electronegativity than asecond metal oxide film of the plurality of metal oxide films, and thefirst metal oxide film is closer to the interior surface of the trenchthan the second metal oxide film, and the second metal oxide film of theplurality of metal oxide films has greater electronegativity than athird metal oxide film of the plurality of metal oxide films, and thesecond metal oxide film is closer to the interior surface of the trenchthan the third metal oxide film, wherein each of the first metal oxidefilm, the second metal oxide film, and the third metal oxide film isstacked up from the substrate in a contiguous and conformal manner. 20.The image sensor device of claim 19, wherein the first metal oxide filmis an aluminum oxide layer, the second metal oxide film is a hafniumoxide layer, and the third metal oxide film is a tantalum oxide layer.